Optical fibers can be used to transport and/or enhance signals at certain wavelengths. For example, pump energy at a wavelength λp can be emitted by an energy source, such as a laser, and coupled into an optical fiber having a core containing an active material that interacts with the pump energy, and undergoes certain electronic transitions to form energy at a different wavelength λout. The optical fiber can include, for example, a pair of reflectors that form a lasing cavity at the wavelength λout so that the optical fiber can be used as a laser that converts energy at λp to energy at λout.